what are the main differences?

The main difference is the engine.
The 350 has a 3.5L V6 which produces 268 hp while the 550 has a 5.5 liter V8 which produces a healthy 382 hp.

Other than that, there are some styling differences. The 550 has more aggressive, sporty styling inspired by the AMG model which includes wheels skirts and front and rear bumpers